Matrix Analysis of Warped Stretch Imaging.

Sensitive and fast optical imaging is needed for scientific instruments, machine vision, and biomedical diagnostics. Many of the fundamental challenges are addressed with time stretch imaging, which has been used for ultrafast continuous imaging for a diverse range of applications, such as biomarker-free cell classification, the monitoring of laser ablation, and the inspection of flat panel displays. With frame rates exceeding a million scans per second, the firehose of data generated by the time stretch camera requires optical data compression. Warped stretch imaging technology utilizes nonuniform spectrotemporal optical operations to compress the image in a single-shot real-time fashion. Here, we present a matrix analysis method for the evaluation of these systems and quantify important design parameters and the spatial resolution. The key principles of the system include (1) time/warped stretch transformation and (2) the spatial dispersion of ultrashort optical pulse, which are traced with simple computation of ray-pulse matrix. Furthermore, a mathematical model is constructed for the simulation of imaging operations while considering the optical and electrical response of the system. The proposed analysis method was applied to an example time stretch imaging system via simulation and validated with experimental data.

Enhancement of ultraviolet-beam generation through a double walk-off compensation.

We propose a novel walk-off-compensation method for ultraviolet-light-source development. A walk-off occurring in the frequency mixing of infrared and green beams is doubly compensated for using an external walk-off compensator and a nonlinear-material set with internal walk-off-compensation arrangement. We theoretically and experimentally verified that our method can improve the power and beam shape of the output ultraviolet beam.

Femtosecond laser and arc discharge induced microstructuring on optical fiber tip for the multidirectional firing.

Most optical fibers are designed for forward firing i.e. the light is emitted at the distal end along the optical axis of the fiber. In some applications such as the laser surgery and laser scanners, side firing of the optical fiber is required. In this paper, we present the microstructuring of an optical fiber tip using the femtosecond laser and an arc discharging process for the multidirectional firing of the beam. The distal end of the optical fiber with diameter of 125 µm was machined into a conical structure using a femtosecond laser. The surface of the machined tip was exposed to the arc discharge using a fiber splicer. The arc discharge leads to the melting and re-solidification of the fiber tip. This results in a smoothing of laser-induced conical microstructure at the tip of the fiber. We were able to demonstrate the multidirectional (circumferential) emission of the light from the developed fiber tip.

Bandwidth control of a Ti:PPLN Solc filter by a temperature-gradient-control technique.

We have demonstrated the bandwidth control of a Ti-diffused periodically poled LiNbO(3) (Ti:PPLN) Solc filter by a temperature-gradient-control technique. Up to 2.8 nm of filtering bandwidth was achieved with a simple temperature-gradient-control technique in a 78-mm-long of Ti:PPLN waveguide, which has a 0.2 nm filtering bandwidth at an uniform temperature. We have also analyzed the experimental results with the theoretical calculation which is derived from the codirectional coupled mode equations.

Single-body lensed photonic crystal fibers as side-viewing probes for optical imaging systems.

We report the fabrication and performance of a lensed photonic crystal fiber (PCF) designed as a compact but effective side-viewing optical imaging probe. The lensed-PCF probe was implemented in a single body without using any other fibers or additional optics. The beam expansion region and a focusing ball lens, necessary for a focuser, were simultaneously formed along a small piece of PCF by applying arc discharges. The side-viewing ability was provided by polishing the ball lens with a femtosecond laser to form a total internal reflection surface. The working distance and the transverse resolution of the fabricated single-body lensed-PCF were experimentally measured to be 570 and 6.8 microm, respectively. With the proposed lensed-PCF probe, optical coherence tomography images of an in vitro biological sample were successfully obtained.

Bandwidth-tunable band-rejection filter based on helicoidal fiber grating pair of opposite helicities.

We have developed a new type of all-fiber band-rejection filter consisting of a helicoidal long-period fiber grating pair of opposite helicities, which provides highly flexible spectral control over the rejection bandwidth. The detailed fabrication method and operation principles of the proposed bandwidth-tunable band-rejection filter are described. The proposed device enables unique rejection bandwidth tuning over more than 14 nm at the rejection level of 15 dB, with low insertion loss and polarization-dependent loss achieved by adjusting torsion stress.

100-kHz high-power femtosecond Ti:sapphire laser based on downchirped regenerative amplification.

We have generated femtosecond pulses with a peak power as high as 0.7 GW at the repetition rate of 100 kHz from a downchirped-pulse amplification (DPA) Ti:sapphire laser. For the high-energy amplification with high repetition rate, we employ a regenerative amplifier, acousto-optically switched and pumped by a Q-switched green laser. The DPA-based dispersion control is achieved up to the third-order term by use of a prism-grating stretcher and a glass compressor. We have obtained 28 muJ, 39 fs laser pulses with a compression efficiency of 95 %. Temporal and spatial characteristics of the laser pulses are investigated.

All-optical AND and NAND gates based on cascaded second-order nonlinear processes in a Ti-diffused periodically poled LiNbO(3) waveguide.

All-optical AND and NAND gates have been demonstrated in a Ti-diffused periodically poled LiNbO(3) channel waveguide which has two second-harmonic phase-matching peaks by cascaded sum-frequency-generation/difference-frequency-generation (cSFG/DFG) and sum-frequency-generation (SFG) processes. The conversion efficiency of signal to idler (AND gate signal) was approximately 0 dB in cSFG/DFG process. In the second SFG process, more than 15 dB extinction ratio between signal and dropped signal (NAND gate signal) has been observed.

Tunable Q-switched erbium-doped fiber laser based on digital micro-mirror array.

We propose and demonstrate a tunable Q-switched erbium doped fiber laser with a digitally controlled micro-mirror array device. The tunable and pulsed output of the laser was achieved by the pixelated spatial modulation of the micro-mirror array. The wavelength tuning from 1530 nm to 1555 nm was shown with wavelength selectivity of ~0.1 nm and the pulsed operation was accomplished with 130 Hz repetition rate.

All-optical wavelength conversion and tuning by the cascaded sum- and difference frequency generation (cSFG/DFG) in a temperature gradient controlled Ti:PPLN channel waveguide.

All-optical single and multiple wavelength conversion and tuning by the cascaded sum- and difference frequency generation (cSFG/DFG) have been demonstrated in a temperature gradient controlled periodically poled Ti:LiNbO3 (Ti:PPLN) channel waveguide. Up to 4 channels of wavelength division multiplexed (WDM) signals which have 100 GHz channel spacing were simultaneously wavelength converted at a 16.8 degrees C temperature difference between both end faces in a Ti:PPLN waveguide. The 3 dB signal conversion bandwidth was measured to be as broad as 48 nm at single channel conversion. The maximum wavelength conversion efficiency and optical signal to noise ratio of wavelength converted channel were approximately -16 dB and -20 dB at a total pump power level of 810 mW.

Channel-selective wavelength conversion and tuning in periodically poled Ti:LiNbO(3) waveguides.

All-optical wavelength-selective single- and dual-channel wavelength conversion and tuning has been demonstrated in a periodically poled Ti:LiNbO(3) waveguide that has two second-harmonic phase-matching peaks by cascaded sum and difference frequency generation (cSFG/DFG). The wavelength conversion efficiency was measured to be -7 dB with coupled pump power of 233 mW.

Broadening of the second-harmonic phase-matching bandwidth in a temperature-gradient-controlled periodically poled Ti:LiNbO3 channel waveguide.

We have demonstrated broadening of the phase-matching bandwidth in a periodically poled Ti:LiNbO3 (Ti:PPLN) channel waveguide Lambda = 16.6 microm) by using a temperature-gradient-control technique. With this technique, we have achieved a second-harmonic phase-matching bandwidth of more than 13 nm in a 74-mm-long Ti:PPLN waveguide, which has a 0.21-nm phase-matching bandwidth at a uniform temperature.